This invention relates to the field of seals and gaskets for blocking gaps between conductive structures against passage of electromagnetic interference (EMI), while also sealing the gap environmentally against airflow, noise, dust or the like. Specifically, this invention relates to a new gasket that has a conductive compressible core.
Electromagnetic interference (EMI) has been defined as undesired conducted or radiated electrical disturbances from an electrical or electronic apparatus, including transients, which can interfere with the operation of other electrical or electronic apparatus. Such disturbances can occur anywhere in the electromagnetic spectrum. Radio frequency interference (RFI) is often used interchangeably with electromagnetic interference, although it is more properly restricted to the radio frequency portion of the electromagnetic spectrum, usually defined as between 10 kilohertz (KHz) and 10 gigahertz (GHz). Comprehensive technical summaries are available from a number of sources.
A shield is defined as a metallic or otherwise electrically conductive configuration inserted between a source of EMI/RFI and a desired area of protection, which has the capability of absorbing and/or reflecting EMI/RFI and reducing the energy levels thereof. As a practical matter, such shields normally take the form of an electrically conductive housing that is electrically grounded. The energy of the EMI/RFI is thereby dissipated harmlessly to ground. Such a shield may be provided to prevent EMI/RFI radiating from a source or to prevent EMI/RFI (generated randomly or by design) from reaching a target, or both. Most such housings necessarily have access panels, hatches, doors and/or removable covers.
The gaps between the panels, hatches, etc., and the housing provide an undesired opportunity for EMI/RFI to pass through the shield. The gaps also interfere with electrical currents running along the surfaces of the housings from EMI/RFI energy which is absorbed and is being conducted to ground. The gaps reduce the efficiency of the ground conduction path and may even result in the shield becoming a secondary source of EMI/RFI leakage, from gaps that act as slot antennas.
Various configurations of gaskets have been developed over the years to close the gaps of such shields and to effect the least possible disturbance of the ground conduction currents. Each seeks to establish as continuous an electrically conductive path as possible across the gap(s). However, there are inevitable compromises between (1) the ability of the gasket to smoothly and thoroughly engage and conform to the surface of the housing adjacent the gaps, (2) the conductive capacity of the gasket, (3) the ease of mounting the gasket, (4) the ability of the gasket to withstand abrasive wear and tear, as well as repeated compression and relaxation, and (5) the cost of manufacturing the gasket.
The following patents are illustrative of the kinds of gaskets that have been proposed to prevent EMI/RFI leakage.
An RFI shielding gasket disclosed in U.S. Pat. No. 3,555,168 is formed as a conductive foil lamina bonded to a resilient foam backing by a flexible adhesive and is mounted by a pressure-sensitive adhesive on the back of the foam backing. The gasket is a flat member produced from flat layers of flat stock, rather than by extrusion or molding. In a preferred embodiment, the foam is a closed cell, medium density neoprene foam from 0.015 to 0.500 inches thick. The resulting laminate is die cut to shape, and is said to be RFI tight and dust tight. A seal disclosed in U.S. Pat. No. 3,312,769 has a resilient core, preferably neoprene sponge, surrounded by a metallic mesh, preferably an alloy of nickel and copper such as Monel. There is no indication the core is bonded to the metallic mesh in any fashion.
A sealing gasket disclosed in U.S. Pat. No. 2,477,267 comprises a resilient gasket having a network of electrically conductive wires embedded therein and throughout, the wires having portions exposed on opposite surfaces, of the gasket.
A seal disclosed in U.S. Pat. No. 3,466,906 comprises a body of resilient plastic foam material having a plurality of interconnected open cells and a coating of electrically conductive material provided throughout the body on the surfaces of the plastic elements. A conductive coating is preferably applied by electroplating to form a conductive surface on the seal. The seal is available as resilient, single and dual, all-metal strips or compressed shapes. The seal is also available with an elastomer core, in round or rectangular profiles, the core being solid or hollow.
Another gasket is disclosed in Buonanno, U.S. Pat. No. 4,857,668. The sheath is bonded to the foam core as an integral part of a continuous molding process in which the foam is blown or expanded within the sheath. As the foam cures, a sealed outer boundary layer forms on the surface thereof which faces the inner surface of the sheath. The outer boundary layer is stated to have an adhesive character that effects a strong bond between the foam core and the sheath. An adhesive strip may be attached at the overlapped edges at the outer edge thereof for mounting the gasket to a substrate.
One feature of prior art gaskets is that an electrical current had to traverse a path along an outer conductive sheath. To maintain a short electrical path a manufacturer would wrap the non-conductive foam core with an adhesive backed conductive film or fabric. This wrapped material was then die cut or laser cut to allow electronic connectors to pass through the gasket. The cut outs for the electrical connectors that are closer to the center of the gasket have a longer electrical path to follow than those closer to the edge. Thus, the shielding is less effective for these connections. To shorten this electrical path conductive films or fabrics were taped around the cutouts. This method was costly because manufacturers were unable to automate the process.
An object of the invention was to develop a compressible gasket that would prevent EMI and RFI and also had volume conductivity while maintaining the shortest electrical path across the gasket.
This and other objects of the invention are accomplished by a gasket for shields housing electrical apparatus and the like, operation of which tends to generate or be affected by, EMI/RFI, comprising: a conductive, compressible core, an electrically conductive sheath, and means for mounting the gasket.
The core may be made from polyester, nylon, Basofil, or other material that can be plated with metal or other conductive mediums.
Another embodiment comprises a sheath. The sheath is preferably a flexible, electrically conductive and substantially abrasion resistant sheath surrounding the core and bonded core. The shield may be sealed against EMI/RFI leakage, audible noise emission and environmental infiltration through perimeter gaps of electrically conductive doors, access panels and the like by the actions and interactions of the sheath, the core and the boundary layer. The flexible sheath is continuously pressed into positive and thorough contact with the conductive surfaces, between which it is mounted, by the resilient compressible core, forming a continuous electrical path across the gaps and preventing EM/RFI leakage through the gaps.
The sheath is preferably a fabric, formed at least in part from electrically conductive fibers, or coated with an electrically conductive layer, or both. The metal surface, formed for example by electroless plating or sputtering, is not only highly resistant to damage from abrasion and the like, but is characterized by a relatively low coefficient of friction which enables the gasket to withstand sliding frictional contact. This provides an opportunity to mount the gasket in positions that are not appropriate for gaskets unable to withstand the rigors of sliding contact.
These and other objects and advantages of the invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiments of the invention, shown in the accompanying drawings.